Program distribution system



May 28, 1935'. E. l. GREEN y 2,002,603

PROGRAM DI STRIBUTION SYSTEM Filed March 10,'1953 2 sheets-sheet 1 Jg B IDI-agrari: Center enfra/ fce Voice Circuit lATToRNl-:yf

May 28, 1935. E. l. GREEN 2,002,603

PROGRAM DISTRIBUTION SYSTEM l Demozi. .Ef-77,0. S-ggg',

INVENTOR y EIGl/:een/

ATTORNEY Patented May. 28, 1935 PATENT OFFICE PROGRAM DISTRIBUTION SYSTEM Estill I: Green, East Orange, N. J., assignor to American Telephone and Telegraph Company, a corporation of New York Application March 10, 1933, Serial No. 660,321

Claims.

This invention is concerned with systems of distributing programs over wire circuits to subscribers, andmore especially with systems in which the programs are distributed at carrier frequencies.

Itis a particular object of the invention to secure a carrier frequency distribution system which may be applied to a distributing network of the type commonly used for telephone purposes. .A further object of the invention is to avoid the use of a large and expensive wire plant for distributing the carrier frequency programs.

A feature of the invention is the provision of l apparatus for obtaining the carrier frequency program bands at a central point, which point serves a large distribution area so that duplication of this apparatus may be avoided.

Yet another feature of the invention is the provision of means whereby a plurality of carrier frequency program bands `and their associated carrier frequencies may be eiiiciently quencies are transmitted through one amplifier,

amplified.

These and other objects and features of the invention will now be more readily'understood from the following description when read in connection with the accompanying drawings, in which Figure 1'il1ustrates diagrammatically a carrier frequency system of distributing programs to subscribers; Fig. la shows an arrangement of apparatus for receiving both wire programs and radio programs; Fig. 1b shows an arrangement for selecting carrierfrequencies; Figs. 2 and 3 show arrangements of apparatus which lmay be used in the system of Fig. 1 to produce the carrier frequency program bands; and Fig. 4 shows a distribution system which may be substituted for a part of the system of Fig. 1.

The system shown in Fig. 1 'comprises sound pick-up apparatus PA, consisting, for example, of a microphone, amplier, volume control,v and associated apparatus, whereby eachof various sound programs representing speech, music, or the like, may be picked up and transmitted over v pick-up circuits `PL to the program center. Some of the programs may originate in'studios located at the program center, while others may be received from considerable distances over suitable transmission circuits. l.

At the program center, apparatussuch as indicated by CM is provided whereby ,each program band is shifted to a position in y the carrier frequency range. The apparatus CM is j designed to yield substantially a single side band output with the carrier frequency suppressed in the side band output and delivered separately. 'This apparatus for delivering separately the sin- A(Cl. 179-15) gle side band program and its carrier frequency is one of the important parts of the system and will be referred to in greater detail in connection with Figs. 2 and 3.

The side bands from several different programs are combined and passed through the common anmplier SA1. While the figure shows three different program channels, it is obvious that a smaller or greater number may be employedl if desired. After amplification, the side 10 bands are transmitted over suitable trunk circuits ST to qvarious intermediate distribution points, which may, if desired, be telephone central offices. While it would be possible to connect several central offices in parallel on single trunk circuit, the usual arrangement would be to have a separate trunk to each'central oiiice. The three carrier frequencies are amplified together'in the amplier CA1, and are then transmitted over trunks CT to the central omces.

At each central office the three carrier fre- CA2, and the three side bands through another amplifier, SAQ, the carriers and side bands being then combined for transmission to the subscribv ers. This feature of separately amplifying the 25 carrier and side bands at the central oices and the program center is another important feature of the invention which will be considered in detail later.

In the arrangement of Fig. 1 the combination of 30 carriers and side bands is applied to each of a number of subscribers lines which may be used also for regular telephone service. The side bands and carriers are connected to each subscribers line SL through two series condensers HC1' and HG2. These condensers, together s with an inductance HL; which is connected across the common carrier frequency bus, comprise a simple high pass 'filter arrangement. The function of this filter is to avoid bridging loss to the voice-frequency currents transmitted over the subscribers loop and to prevent the occurrence of voice-frequency cross-talk between diierent subscribers telephone circuits through the medium of the common connection at the carrier bus.

On the central office side of the junction of the high frequency circuit with the subscribers loop there is a simple low pass filter LP1 comprising a series inductance LL1 and' a shunt capacity LCr. The usual low frequency currents pass through this filter to or from the regular central office equipment. This low pass lter serves to attenuate any carrier frequency components which might accompany the voice-frequency'4 currents 55 coming from the .central ofce and which wo'd constitute interference in the carrier system. Such interfering components, for example, might be harmonics of ringing currents, busy tones, etc.

At the subscribers station a low pass filter LPz provides a path for the normal telephone currents to enter or leave the subscribers telephone set TS and eliminates any carrier interference which may be generated in the telephone set. This low pass filter may include series inductances LL: and LLa and a shunt condenser LCz. The carrier frequency circuit, which is connected in parallel with the subscribers voice frequency circuit, includes.v

series condensers HCa and HCl which serve to block the direct currents, normally used on the subscribers loop, and also to present a high impedance to the voice frequency currents. On Dassing through these condensers the program bands are passed by the transformer HTto an adjustable band filter AB by means of which any of the available program bands may be selected. This variable band filter may consist 'of a T network of impedances, including two adjustable series arms, each comprising in series an adjustable condenser AC and inductance FL, and a shunt arm comprising in parallel a fixed condenser FC and a tapped inductanceAL. A single control may be provided for"adjustingbothl the .series condensers and the shunt inductance.

After passing through the band filter the side band and carrier frequency of the selected program are applied to a demodulator PD which is connected to an amplifier PA and a loud speaker LS. The amplifier and loud speaker may be units employed exclusively for the carrier frequency program system or they may be employed Valso as the amplifier and loud speaker of the subscribers radio set. An arrangement of this latter type is illustrated in Fig. 1a which shows how the program circuit may be connected from the demodulator PD through the step-up transformer LT, the plug PG, and the jack JK to the audiofrequency amplifier input of the radio set. Also, if desired, the detector in the subscribers radio set may be employed as a demodulator for the carrier system.

In the system as outlined above there are two features which are of particular interest:

1. The performance at the program center of the difficult operation of obtaining the desired single side bands, and the use of carrier distribution from the program center to the various intermediate distributing points or central offices.

2. The separate amplification of carriers and side bands both at the central offices and at the program center.

The single side band feature will first be considered. It is now well understood that in order to obtain high quality reproduction, it isdesirable to include both the high and low frequencies in a program channel, and for best results a program band extending from about 30 cycles to 10,000 cycles or higher may well be desired. With double side band transmission a program band of this width would require a carrier channel which is 20,000 cycles wide. Now it is diicult to select program bands of such width in any simple type of band filter which would be economical for use at the subscribers premises. Furthermore, with several channels of such width spaced at the frequency interval required for selection between them without mutual interference, the total frequency range which the subscribers loop must be capable of handling becomes excessive. Hence, it is extremely advantageous to employ substan- -with a, single band filter.

tially a single side band for each program channel. v

The modulating apparatus for converting each voice frequency program band into a carrier side band might be located either at a program center as shown in Fig. 1 or ateach central office. However, as will be seen presently, this modulating apparatus is quite 4complicated in nature, so that it is desirable to make one set of such apparatus at the'program center serve for distribution over a wide area. With this arrangement the transmission between the program center and` each central office is at carrier frequencies.

l'I'he reason for wishing to secure the single side bands atI the program center being clear, the method of accomplishing this result may next be considered. In a carrier frequency program system it is difficult to use the scheme, ordinarily employed in carrier systems which furnish telephone channels, of separatingout the wanted side band and -eliminating the vunwanted side band This difficulty arises from the fact that "in a program system it is necessary to transmit the very low frequencies of the vvoice band, perhaps down as far as 30 cycles, whereas inthe telephone case a system which cuts of the voice frequencies below about 250 cycles is usually satisfactory. Consequently, in the output of a program modulator the unwanted side band is within 60 cycles of the wanted side band, and it is difficult, if not impracticable, to build a `carrier frequency band filter which will attain the desiredsuppression quencyspace. .f v

An arrangementwhich may be used in the carrier frequency program system for obtaining a single side band is shown in Fig.,2. This arrangement may be used as that part of Fig. 1 included between the lines A-A and B-B. The program band, extending from, for example, .03 to 10 kilocycles, is applied to a modulator PM1 which is furnished with a carrier frequency supply from the source MC1. Let it be assumed, for example, that the frequency of this carrier supply i. 25 kilocycles. The modulator output will then include an upper side band extending from 25.03 kilocycles to 35 kilocycles and a lower side band extending from 24.97 kilocycles to 15 kilocycles. The carrier frequency may be suppressed in a balanced or push-pull type `of modulator, of the type disclosed in the patent to J. R. Carson, No. 1,343,307, dated June 15, 1920. The band filter BF; passes the frequencies of the lower side band and suppresses the greater part of `the upper side band, as well as other unwanted frequencies, leaving partially attenuated frequencies of the upper side band which extend upward from 25.03 kilocycles for va short distance. If these partially attenuated frequencies are not eliminated in some way, they will, upon demodulation, coincide with the corresponding frequencies in the lower side band and according to their phase relations will add to or subtract from the frequencies of the lower side band. In generaL'the phase relations will besuch as to give an irregular transmission characteristic in the frequency range of the overlap.

The unwanted frequencies may, however, be eliminated by the method shown in Fig. 2. In a second modulator PM2, also of the carrier suppression type, the Wanted side band and the partially suppressed frequencies of the unwanted side band are brought down to a low frequency position where the unwanted frequencies may be in such a' narrow freeliminated by a band filter BF2. The frequency supplied to the second modulator is derived from a source MC2. This frequency is made somewhat higher than the previous carrier frequency so that it has the effect of inverting the band and bringing the lowest frequency of the wanted side band to a position where the unwanted frequencies may readily be'eliminated. In the example shown, the second carrier frequency is assumed to be 26 kilocycles so that the band filter BF2 is designed to pass frequencies extending from 1.03 to 11 kilocycles while attenuating frequencies extending downwards from .97 kilocycle. This lter BF2 likewise suppresses the frequencies representing the sum of the input carrier and signal bands applied to the modulator PM2. These latter frequencies evidently extend upward from 41 kilocycles.

The program band in the output of the filter BF2, which is located between the frequencies of 1.03 and 11 kilocycles, is shifted to its final position by a third step of 'modulation in the modulator PMs, also of the carrier suppression type. It is assumed that a carrier frequency of 26 kilocycles is applied to this modulator from the same source MC2 employed for the second modulator. In the output of the modulator PM3 the band filter BF2. selects the wanted program side band extending from 15 to 24.97 kilocycles and suppresses the unwanted side band, as well as other unwanted components. Thissuppression of the unwanted side band is readily accomplished since the unwanted side band is now 27.06kilocycles away from the wanted side band. 'Thus there is ,obtained a true single side band without accompanying frequencies of thel unwanted side band and without the carrier frequency.' A supply of the carrier frequency for transmission to the central offices may be obtained separately from the carrier frequency source MC1.

Another scheme which might be used in place of the one just described is illustrated in Fig. 3. In this instance two side bands are used to transmit a small part of the program band and one side band for the remainder of the program. As shown in the figure the original program band which again is assumed to extend from .03 kilocycle to 10 kilocycles is split into two parts by the high and low pass filters HF and LF. The lower part of the band is assumedto extend from 30 to 500 cycles and the upper part from 500 to 10,00() cycles. At the suppression frequency of 500 cycles a sharp division of the band can be secured. The two parts of the band are applied v to separate balanced modulators HM and LM which are furnished with a carrier frequency (assumed in this case to be 25 kilocycles) by the source MC4. The output of the modulatorv HM is applied to a band filter BF4, which passes a wanted band extending from 15 to 24.5 kilocycles `and attenuates the unwanted band and other unwanted frequencies. In the output of the modulator LM; which may be'of the balanced type, thereby suppressing the carrier frequency, the filter BFs passes both side bands, i. e. the range from 24.5 to 25.5 kilocycles, while attenuating other unwanted frequencies which may be present in the output. The double sideband from 0 the filter BF5`is combined with the single side It is evident that upon demodulation with the proper carrier frequency the two side bands corresponding to the lower part of the program range will coincide. The transmission eiciencies of the single side band and double side band parts of the modulator system in Fig. 3 may be so adjusted that a uniform frequency characteristic will be obtained after demodulation.

In case it is n'ot desired to separate out the carrier frequency, this may be allowed to pass through the modulator LM along with the products of modulation and together with the double side band from 24.5 to 25.5 kilocycles selected by the filter BF5 and combined with the remaining side band components. I

There will now be considered the scheme of amplifying the carriers and the side band separately both at the program center and the central office. In any system for distributing program material from a central point to a. number of subscribers it is usually desirable that the volume applied ,to each subscribers line should not fall below a certain minimum value generally determined by interference considerations, and it is also desirable to be able to connect a large number of lines to a common amplifier. Hence'the carrying capacity of thecentral. office amplifier becomes a matter of considerable importance.

In such a situation the amplification of the carriers and side bands in separate amplifiers as disclosed is advantageous because it permits more effective utilization of the amplier carrying capacity. The load capacity of the amplifier used for the carrier frequencies is relatively high since any unwanted frequencies resulting ,from distortion or non-linearity may, if necessary, be eliminated by filters or tuned circuits in the output'as shown in Fig. 1b. In the side band amplifier, the'interchannel cross-talk due to intermodulation effects is very much less than if the larger `carrier voltages were transmitted through it. It

appears that the total number o f' subscribers lines that may be` fed from two amplifiers when used in the manner outlined above is of the order of double the number which may be fed from the same two amplifiers when each amplifier transmits both carriers and side bands.

In order to avoid the dificulty of amplifying a carrier frequency and a side band in a common amplifier, systems in which the carrier frequency is not transmitted over the line have been developed. In the present instance a carrier suppression system would be desirable were it not for the'diiculty of resupplying the carrier frequencies at the subscribers premises. The scheme which has been described provides in effect a carrier suppression system as far as the subscribers lines and an auxiliary system for distributing the carrier frequencies to the subscribers lines. This arrangement is also advantageous from the standpoint of testing, since it makes it possible for testing purposes to select each side band by means of a simple variable lter of the same type as that shown at the subscribers premises in Fig. 1, and thus to measure the program volume with a suitable voltmeter or volume indicator. If the carrier frequencies were present with the side bands at the testing point, it would be difficult to separate out' the-carriers in order to secure the side bands which alone are an index of the volume.

Instead of transmitting the programs over subscrib'ers loops used for telephone service, it would be possible to employ a separate distribution network exclusively for bringing programs to subscribers. If this were done it might be desirable to cut down the total number of circuits required by connecting a number of subscribers in parallel on a single circuit. A party-line arrangement of this kind is shown in Fig. 4,. which may be substituted for that part of Fig 1 lying to the right of the line C-C. At the central office the programs are applied to a number of circuits such as SL', each of which feeds several subscribers. In order to avoid reflection effects on the circuit, and possible interaction of one subscribers station on another, the main circuit is terminated in an impedance Z which may be a resistance approximately equalling the characteristic impedance, and each subscribers drop is connected to the main circuit through series resistances SR. The subscribers receiving apparatus may be the same as that shown in Fig. 1.

It will be obvious that the general principles herein disclosed may be embodied in many other organizations widely different from those illustrated without departing from the spirit of the invention as defined in the following claims.

What is claimed is:

l. In a system for the distribution of intelligence, the method of carrier transmission which consists in producinga plurality of carrier frequencies, separately producing a single side band of each of said carrier frequencies, segregating said carrier frequencies from said side bands, transmitting all the carrier frequencies over one transmission line, transmitting all the side band frequencies over another transmission line, amplifying the carrier frequencies at a plurality of locations'along said transmission line, separately amplifying the side band frequencies at the same locations, combining the carrier and side band frequencies, transmitting the combined lfrequencies over a common transmission line and utilizing the received frequencies.

2. In a system for the distribution of intelligence, means for producing a plurality of prov gram bands, means for modulating each program and means at each receiving station for utilizing said carrier and side band frequencies.

3. In a system for the distribution of intelligence, means for producing a plurality of program bands, means for modulating each program band on a different carrier frequency, means for obtaining a single side band from each modulator, means for combining said side bands and transmitting them to a plurality of intermediate points, means for combining said carrier frequencies and transmitting them to the same intermediate points, means at each intermediate point for amplifying the carrier frequencies, means at each intermediate point for amplifying the side band frequencies, means for combining the carrier frequencies and the side band frequencies after amplication, means for transmitting said combined frequencies to a plurality of receiving stations, means at each receiving station for selecting and demodulating said received carrier and side band frequencies,` and means at each receiving station for amplification and reproduction of said demodulated frequencies.

4. In a system for the distribution of intelligence, the method of carrier transmission which consists in producing a plurality of carrier frequencies, separately producing side bands of each of said carrier frequencies, segregating said carrier frequencies from said side bands, transmitting all the carrier frequencies :over one transmission line, transmitting all the side band frequencies over another transmission line, amplifying the carrier frequencies at a plurality of locations along said transmission line, separately amplifying the side band frequencies at the same locations, combining the carrier and side band frequencies, transmitting the combined frequencies over a common transmission line and utilizing the received frequencies.

5. In a system for the distribution of intelligence, means for producing a plurality of program bands, means for modulating each program band on a different carrier frequency thereby producing side bands in relation to each said carrier frequency, means for combining the said side bands and transmitting them to a plurality of intermediate points, means for combining said carrier frequencies and transmitting them to the same intermediate points, means at each intermediate point for amplifying the carrier frequencies, means at each intermediate point for amplifying the side band frequencies, means for r combining the carrier frequencies and the side band frequencies after amplification, means for transmitting the combined frequencies to a plurality of receiving stations and means at each receiving station for utilizing said carrier and side band frequencies.

ESTILL I. GREEN. 

